I can't wait to see the newest version of The Other Boleyn Girl. The most recent scholarly studies have shown that indeed the two sisters were very close in age and that is may very well be that Mary was the younger. People assume she was older because she was Henry's first choice of the two, but he did not discriminate so much. He would have any wench in the kingdom he wanted and could not be denied, so he probably went for whichever one warmed up to him the most or grabbed his attention. It is interesting that he targeted the Seymour family twice, when there were plenty of other women he could have had. So there was certainly another sort of attraction to these two sisters that grabbed him~possibly a similarity in their looks.
The Tudors period certainly offers a fine array of characters for you to work with, Kat, from Pope to Commoner!